Determining Allowable Positional Tolerances for a Dowel Pin/Slot

[RealSaladsamurai]

I trying to determine if the tolerances on a mechanical drawing make sense with respect to the part function. In the attached image There are two components that are to be welded together. One part has 2 dowels pins press fitted into it while the other part as a hole-slot pair. From my online searches, this seems to be a pretty common scenario. I just want to make sure that I am thinking about the stackup correctly.

I am thinking that in order to do the stackup I should use the fixed fastener formula. Typically, with a fixed fastener scenario, you have one part with the fastener tapped in and the other part has a thru hole. We do not have a thru hole, but instead have a blind hole for the dowel to slip-fit over. I am assuming that we can use the fixed fastener formula, where instead of using the thru hole part thickness P in equation below, we instead use the maximum height that the dowel pin can attain.

So the virtual condition of the fastener can be found by:

VC_F = F + T2*(1+2P/D)

where:

F = MMC diameter of dowel = .5003 (from MS16555-677)
T2 = tolerance of position of fastener = .014
P = max height of dowel pin above surface = .51 (default tolerance of .01 on .XX
D = min depth of pin in hole = .995 (which can't really happen at same time P as at worst case)

So I arrive at:
VC_F = .5003 + .014*(1+2*.51/.995) = .529

The virtual condition of the mating hole is given by

VC_H = H - T1

where:

H = MMC diameter of hole = .5004
T1 = positional tolerance of slip-fit hole = .010

So that:
VC_H = .4904

This would appear to be a big problem since the VC of the hole is .039 less than the VC of the fastener.

Is this the correct way to do this? I realize that the slot has a sloppy tolerance in one direction, but I am not taking advantage of this fact in my calculations. Can somebody help to clarify how I can incorporate this fact? I am not sure how to mathematically account for this "one-sided" aspect of the slot.

Thanks again for your help.

________________________
FEMAP v11.1.0
MSC Nastran v2013

 

[drawoh]

RealSaladsamurai,

The fixed and the floating fastener formulae are not relevant to dowel pins. The clearances around dowel pins are too small for reliable assembly, which is why one of them is slotted. The round dowel hole locates your part in X and Y. The dowel slot prevents your part from rotating.

In this situation, dowel pins locate your part to whatever was the true position specified on your drawing. Dowel pinned assemblies are very repeatable.

--
JHG

 

[flash3780]

You should consider adding a projected tolerance to the pin hole equal to the length of the pin. That will prevent error in angularity from impeding assembly.

 

[KENAT]

You have a hole and slot - the whole point is that you can have very tight tolerance on the hole to dowel to give very good location.

You can then have tight tolerance on the width (& to some extent orientation) of the slot to give good orientation control

You can even cheat and have looser tolerance on the width so long as one edge is well controlled and you always push on that edge.

The hole pin combo can even be a press fit if appropriate.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[RealSaladsamurai]

Thank you all for your input. Sorry I did not get back to you sooner as I had to switch gears and work another task. Thanks again!

________________________
FEMAP v11.1.0
MSC Nastran v2013